The present invention relates to ink-jet printing, and is more particularly concerned with color printing using a single printhead with overlapping linear arrays of ejectors to emit a plurality of different types of ink from various ejectors therein.
In existing thermal ink jet printing, the printhead typically comprises one or more ink ejectors, such as disclosed in U.S. Pat. No. 4,463,359, each ejector including a channel communicating with an ink supply chamber, or manifold, at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels, a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a print sheet, such as a piece of paper. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink-jet printing is known as "drop-on-demand" printing. Other types of ink-jet printing, such as continuous-stream or acoustic, are also known.
In a single-color ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print sheet, either by moving the print sheet relative to a stationary printhead, or vice-versa, or both. In some types of apparatus, a relatively small printhead reciprocates across a print sheet numerous times in swaths, much like a typewriter; alternatively, a printhead which extends the full width of the print sheet may be passed once down the print sheet to give full-page images, in what is known as a "full-width array" (FWA) printer. When the printhead and the print sheet are moved relative to each other, imagewise digital data is used to selectively activate the thermal energy generators in the printhead over time so that the desired image will be created on the print sheet.
As ink-jet products enter the market, they must respond to consumer demands for color printing, particularly when documents are prepared on personal computers. A common and expectable desired output for a color ink-jet printer would be a document such as a newsletter in which mainly black text will accompany a full-color image, such as a graph, which occupies only a portion of a sheet. There will thus be documents in which black-only (monochrome) and full-color modes may be in demand in the same job, and even on a single sheet. To optimize output speed of a color ink-jet printer, it may be desirable to provide a printer which can operate in two modes automatically as needed, a black-only mode and a full-color mode. It is to be presumed that the monochrome mode can print out faster than a full-color mode, because the monochrome mode will require only one "pass" of a printhead over a portion of a sheet, while the full-color mode requires a number of printheads, each printing one primary color, to pass over the same location of the sheet in the course of a job. However, if the printer is operable in two modes as required by the job, it makes sense that the monochrome mode not be unduly slowed down by the architecture of the system, which also must accommodate a full-color mode. Ideally, both the monochrome and full-color modes should operate at optimum speeds respectively.
It is therefore an object of the present invention to provide an architecture for a full-color thermal ink-jet printer, wherein a full-color mode may be optimally "traded off" with a maximum speed possible in a monochrome mode. Merely to place linear arrays of ejectors parallel to each other on a reciprocating carriage in an ink-jet printer has been found to be unsatisfactory from the aspect of print quality: if one ink is placed adjacent another ink on a sheet before the inks are substantially dried, a "muddy" appearance has been known to result as the liquid inks blend into each other on the sheet. Also, in a reciprocating-carriage printer, it has been shown that the hues of combined inks vary noticeably depending on the order in which the inks are placed on the sheet. Of course, in a case where four parallel printheads are each placed on the carriage, the order in which the inks are placed on the sheet will reverse depending on the direction of motion of the carriage; thus, such color printers having this feature are practically limited to printing in one direction only, which causes a serious limitation to be placed on the printer speed.
U.S. Pat. No. 4,812,859 discloses an ink-jet recording head wherein a plurality of nozzle groups are in communication with individual chambers, each chamber adapted to convey ink of one color. The head is retrofittable in a single-color printer to provide multicolor printing capability. The nozzle groups each duplicate a different longitudinal segment of the single color nozzle column pattern.
U.S. Pat. No. 4,855,752 discloses a method of creating an area of a preselected hue comprising a plurality of printed primary colors, in which the various flaws of primary colors are each offset by a predetermined amount, in order to minimize the visual "banding" effect when the boundaries between the swaths of different colors are coincident.
U.S. Pat. No. 4,967,203 discloses a method of producing a color image in an ink-jet printer wherein successive applications of ink dots are staggered relative to pixel locations such that overlapping ink dots are printed on successive passes of a printhead. Pixels are grouped into superpixels and various combinations of colored ink dots are applied to each pixel within each superpixel in a staggered sequence.
U.S. Pat. No. 5,030,971 discloses a "roofshooter" ink-jet printhead having a common heater substrate having at least two arrays of heating elements and a corresponding number of feed slots. Each nozzle array is isolated from an adjacent nozzle array and each nozzle is lined above a respective heating element of a corresponding heater array. With this construction, multi-color printheads are efficiently arranged on a single wafer.
U.S. Pat. No. 5,057,852 discloses an apparatus and method of producing enhanced four-color images with an ink-jet printer. True black ink is aligned for printing between cyan, magenta and yellow color spots in a full-color image. When a black edge is desired, process black (derived from a combination of primary colors) and the true black ink are both used to produce the pixels along the edge. The patent shows a printhead having nozzles for colored inks positioned in line with each other in the direction of travel of the printhead, with the black ink nozzle being disposed in separate lines.